1. Field of the Invention
The present invention relates to a method for manufacturing a rigid internal gear of a wave gear device in which an internally toothed portion formed with internal teeth and a gear main body are securely joined and integrated.
2. Description of the Related Art
A wave gear device commonly has an annular rigid internal gear, a flexible external gear disposed on an inner side thereof, and a wave generator fitted inside the flexible external gear. The wave generator is commonly elliptically contoured, the flexible external gear is elliptically flexed by the wave generator, and external teeth formed on an external peripheral surface thereof partially mesh with internal teeth of the rigid internal gear. When the wave generator is rotated by a motor or the like, the meshing locations of the two gears move circumferentially. Since there is a difference in the number of external and internal teeth, relative rotational movement is produced between the two gears in accordance with the difference in the number of teeth. Therefore, when one of the gears is fixed in place, a reduced rotational output can be obtained from the other gear.
The wave gear device has few components and is lightweight in comparison with other forms of reduction gears. The rigid internal gear, which is a constituent component thereof, must be made more lightweight in order to reduce the device weight. More specifically, since the rigid internal gear is required to be strong and abrasion resistant, heavy iron or copper materials are used in its manufacture. For this reason, a weight ratio of the rigid internal gear is considerable in comparison with other constituent components. In particular, when the rigid internal gear is integrally formed with a mounting flange for mounting on the motor or an output-side member, or when a housing is integrally formed, the weight is considerable, and the advantage of a lightweight flexible meshing type gear device is compromised.
It has been proposed that a portion of the rigid internal gear in which internal teeth are formed, which require strength and abrasion resistance, be manufactured from a ring comprised of an iron or copper material that has strength and high abrasion resistance, a gear main body portion be manufactured from a ring composed of an aluminum alloy or other lightweight material, and the rings be integrally joined together. Rigid internal gears with such a configuration are disclosed in JP-A 2002-339990 and JP-A 2002-307237.
In a rigid internal gear with such a configuration, considerable torque must be transmitted between the ring for forming the gear main body portion and the ring for forming the internally toothed potion, and the rings must therefore be securely joined.